Anemia is measured according to the amount of hemoglobin, the protein that carries oxygen, in RBCs. The World Health Organization (WHO) defines anemia as less than 13 grams of hemoglobin per deciliter for men, and less than 12 grams of hemoglobin per deciliter for women. Approximately three million Americans suffer from anemia. Women and people with chronic diseases are at highest risk of anemia